Solid wood substrates for use in outdoor applications, such as in exterior building materials of various types, typically are protected from the effects of weathering by painting the wood with a conventional paint or by staining the wood with, for example, a pigmented opaque stain. While coatings which are opaque, or substantially so, can afford good protection against weathering, particularly against the combined effects of moisture and ultraviolet light, they do not allow the natural beauty of the wood, such as the grain of the wood, to be seen. It has long been desirable to provide a coating system for exterior wood which is transparent, or essentially so, and yet still affords excellent protection against weathering. However, this object has been elusive with respect to achieving relatively long term exterior durability in a transparent coating system for wood. For example, wood which has been coated with a conventional, clear coating such as a conventional air dry urethane, alkyd or spar varnish often exhibits signs of deterioration in both the coating and, more importantly, in the underlying wood itself, in as little as one year from the time of application.
It is observed in the article, "Microscale Effects Of Ultraviolet Irradiation And Weathering On Redwood Surfaces And Clear Coatings," Journal Of Paint Technology, Vol. 41, No. 531, page 275 ff, (April, 1969) that,
"Most conventional clear coatings strongly absorb ultraviolet radiation, which leads to their rapid degradation. Then degradation of the wood surface beneath may follow. When clear coatings are used that are transparent to ultraviolet radiation, the coating may be very stable, but the wood surface may then be rapidly degraded. The photodegradation of the wood surface, therefore, may be an important factor in the relatively short life of clear coatings that transmit all or only a portion of the ultraviolet wavelengths of solar radiation." PA0 "Any wood exposed to sunlight and rain will in time lose its original colour. The change is due partly to loss of water-soluble extractives, which occurs quite rapidly, but mainly to the breakdown of lignin and other components in the wood by ultra-violet components in sunlight, and subsequent removal of the breakdown products by rain;" and further with respect to clear varnishes as follows. PA0 "The addition of colorless UV light absorbers to clear finishes has found only moderate success to help retain the natural color and original surface structure of wood . . . Opaque pigments found in paints and stains generally provide the most effective and long-lasting protection against light . . . Even when using relatively durable, clear, synthetic resin varnishes, the weatherproof qualities of the wood - varnish system are still limited because UV light penetrates the transparent varnish film and gradually degrades the wood under it . . . Eventually, the varnish begins to flake and crack off, taking with it fibers of the wood that have been degraded photochemically . . ." [citations to references deleted].
Moreover, it was observed in the aforesaid article, that a silicone resin which "was known to transmit considerably more of the ultraviolet radiation than the two varnishes" (a phenolic varnish and a polyurethane varnish), offered little if any protection from degradation upon exposure to a source of ultraviolet light.
It is taught in the article, "Natural finishes for exterior timber," Pigment And Resin Technology, April 1986, page 10 ff, that
"Fully exposed varnish often has a short life, principally because of the tendency of most types to embrittle by weathering, so that in time they disintegrate under the stresses imposed by a wood substrate. Ultra-violet light can pass through the film and bleach or degrade the underlying wood surface. Many cases of premature varnish failure can be traced to detachment of this degraded wood surface layer; this is often brought about by water getting behind the film where end grain is exposed, for example at joints or unstopped nail holes, or as a result of damage."
"These shortcoming are common to all of the varnishes commonly used. Polyurethanes of the moisture-curing and two-pack types give good service indoors but the stresses within such a [sic] strong coatings are sufficient to cause peeling when the outermost layer of wood under the film0382 become degraded; the remnants of the coating are difficult to strip either mechanically or chemically. Unless there are exceptional requirements for toughness and abrasion resistance, polyurethanes are therefore best avoided outdoors. Marine or yacht varnishes have also not been found satisfactory under conditions of full exposure on buildings."
In the article, "Chemistry of Weathering and Protection," The Chemistry Of Solid Wood, Chapter 11, published 1984 by the American Chemical Society, page 435 ff, it is taught that,
It is an object of the present invention to provide a method for providing a wood substrate with long-term durability against weathering and still allow the natural appearance of the underlying wood to be visible. Other objects of the invention will become apparent to the reader infra.